1. Field of the Invention
The present invention relates to a method of allocating channelization code, and more particularly, to a method of allocating channelization codes using Orthogonal Variable Spreading Factor (OVSF) in a communication system. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for effectively and efficiently allocating channelization codes of OVSF.
2. Discussion of the Related Art
In 3GPP, there continues to be discussions in connection with Enhanced Dedicated Channel (E-DCH) to send high speed uplink transmissions in response to high speed downlink shared channel (HS-DSCH). More specifically, from the discussions of uplink transmission in Rel 99/Rel 4/Rel 5, a consensus was reached to allocate Dedicated Physical Control Channel (DPCCH) to the Q branch while allocating Dedicated Physical Data Channel (DPDCH) to the I branch. Both the control channel and the data channel were modulated using dual channel Quadrature Phase Shift Keying (QPSK).
FIG. 1 illustrates an Orthogonal Variable Spreading Factor code tree. Particularly, transmission of DPCCH in the uplink direction always employs a spreading factor (SF) of 256 (SF=256), and the control channel, DPCCH, is allocated to code 0 (Cch,256,0) on the Q branch. Furthermore, in a case where HS-DSCH is used in transmission, the spreading factor of 256 is used in transmitting HS-DPCCH in the uplink direction. At the same time, the code allocated for transmission also changes based on maximum number of codes available in the uplink DPDCH. In detail, when the number of codes is one, the DPDCH is allocated to the 64th code on the Q branch, when the numbers of codes are 2, 4, or 6, the DPDCH is allocated to the first code on the I branch, and when the numbers of codes are 3 or 5, the DPDCH is allocated to the 32nd code on the Q branch.
The uplink DPDCH is different from the downlink DPDCH in that changes in the amount of data affects how the code is allocated. More specifically, as the amount of data increases, a lower spreading factor is used so as to avoid using multicode in transmitting DPDCH in the uplink direction. For example, with increase in data rate, a lower spreading factor is used from 256 to 4. However, when the use of lower spreading factor reaches SF=4, multicodes are used to transmit high data rate via DPDCH in the uplink direction.
The channelization code of the uplink DPDCH is determined by the spreading factor. If only one code is used, the number of the OVSF code tree relating to SF/4 is used (Cch,SF,SF/4). In addition, if two or more codes are used, the uplink DPDCH is determined not only by OVSF code tree but also the allocation of the I/Q branches. In other words, if the number of multicodes are 1 or 2, then code index 1 of SF=4 (Cch,4,1) is allocated, if the number of multicodes are 3 or 4, then code index 3 (Cch,4,3) is allocated, and if the number of multicodes are 5 or 6, then code index 2 (Cch,4,2) is allocated. FIG. 2 illustrates code allocation of uplink DPCH and HS-DPCCH.
In simultaneously using the Dedicated Physical Channel (DPCH) and E-DCH for uplink transmission, if the same rule which applies to DPCH is used, a problem can arise from simultaneously occupying the same OVSF code. Furthermore, if E-DCH and DPCH are simultaneously transmitted, the code allocation scheme of E-DCH is necessary to prevent code occupancy problem.